For safety reasons, motor vehicles are increasingly being fitted with TPMSs.
These monitoring systems are conventionally provided with both:                electronic units (also called “wheel units”) that are mounted on each of the wheels of the vehicle and incorporate sensors for measuring parameters such as the radial acceleration of the wheel and the pressure and temperature of the tire with which this wheel is fitted, and further include a microcontroller, a memory and a radiofrequency emitter; and        a central unit (not shown, mounted on the vehicle) for receiving the signals emitted by the radiofrequency emitters of each wheel, including an electronic computer (or ECU, for electronic control unit), incorporating a radiofrequency receiver connected to an antenna.        
These electronic units periodically provide the central unit with measurements of basic parameters pertaining to each tire—pressure, temperature, acceleration, etc. These measurements may then be exploited in target applications, in particular:                automatic learning by the sensors or tracking the position of the sensors themselves;        locating the wheels so as to monitor the pressure of the tire corresponding to each located wheel;        detecting tire overload and tire wear.        
The measurements, taken by dedicated sensors, are converted and transmitted to a microprocessor incorporated into the central unit via transceiver devices provided with antennas and a CAN bus. The microcontroller of the electronic unit transmits these pressure, acceleration and temperature data via a radiofrequency emitter module to the central processing unit of the TPMS with which the vehicle is fitted. Conversely, this central processing unit transmits low-frequency (LF) control data to the microcontroller. The set of implemented means (sensors, central unit, communication network using the emitters/receivers/transceivers) forms a tire-pressure monitoring system known conventionally by the abbreviation TPMS.
The numerical data delivered by the electronic units to the central unit represent successive levels of variation. From these values, the central processing unit may, after filtering and sampling, formulate pressure, temperature, wear and/or overload signals for the purpose of transmitting these to the vehicle onboard computer.
Tire monitoring is carried out several times over a journey and even when the vehicle is at standstill. The batteries for supplying power to the TPMSs are therefore frequently called upon.
Thus, one important aspect of the use of TPMSs relates to energy consumption, in particular to the energy consumption of the electronic units. Specifically, these electronic units, which use wireless communication means for long periods of time, are supplied with power by energy sources of limited capacity. Generally, the energy required for the electronic units to operate is supplied by a button cell battery. This battery includes a fixed charge guaranteeing the operation of the electronic unit for a mean duration of 10 years.
The main drawback of the batteries for electronic units is that just the internal leakage current of the battery represents nearly 30% of its capacity, even though the battery is oversized so as to be able to guarantee an average service life of 10 years (which nowadays corresponds to a duration longer than the service life of a tire). Thus, the battery represents 20% of the total weight of the electronic unit, which negatively affects the mechanical performance of the electronic unit.
Moreover, the energy capacity of the battery determines the number of functions that may be managed by the electronic unit, such that limiting the energy capacity of the battery limits the number of applications that may be provided by TPMSs. Furthermore, it is difficult to reduce the use of the applications that may be provided by TPMSs without negatively affecting their monitoring function.
Lastly, the cost of the battery for the electronic unit contributes substantially to the total cost of said wheel unit.
One palliative solution, described in particular in the patent US2004/0078662, incorporated herein by reference, consisted in envisaging the replacement of the button cells with power supply means including a piezoelectric element that-is able to transform the mechanical energy from the rotation and vibrations of the wheels into electrical energy.
According to this principle, the power supply means produce renewable energy, and the service life of the wheel units is therefore theoretically unlimited.
However, the amount of energy produced by piezoelectric elements is relatively small and, currently, this amount of energy is too small to supply sufficient power to the various electronic components that are incorporated into the wheel units, in particular when it comes to obtaining acceptable periodicity, i.e. a sufficiently high periodicity, of the sequences for measuring the operating parameters of the wheels.